1. Field of the Invention
The present invention relates to the apparatus for cleaning of sheet metal using a liquid bath, and more particularly to the tanks used for pickling sheet metal.
2. Description of the Related Art
Pickling is the process of removing oxides from steel by converting the oxides to soluble compounds using acids, alkaline solutions, or molten alkali salts. The steel, or other metal, is immersed in a liquid solution (bath) in a tank to remove the oxides, in the form of scales, which develop because of the hot rolling of steel or the changing temperature and moisture conditions from storage. Acid-pickling solutions for ferrous metals typically contain inhibited sulfuric (H.sub.2 SO.sub.4), hydrochloric (HCl) or phosphorous acids. The parameters which may be controlled in the pickling operation include liquid temperature, concentration of the acid, time in the liquid bath, and turbulation. If any of these variables increase, the rate of oxide removal should also increase. Unfortunately, however, higher temperatures may generate corrosive fumes and rapidly corrode the pickling tanks.
The pickling of steel takes place in tanks, i.e., a large liquid-tight open-top tub. A conventional structure for such a tank uses an outer shell of mild steel which is lined with an inner shell or brick or concrete. The inner brick shell is lined with acid-resisting rubber or asphalt and has an inner heavy lining of vitrified acid-proof brick, or tiles set in cement. Consequently, tanks of almost any dimension can be built, particularly those with a rubber lining.
Natural and synthetic rubber are very useful in the construction of acid-resisting pickling tanks because tanks of brick, concrete, or steel can be lined with rubber on site. Natural rubber is the most widely used tank lining, due to its slow aging characteristics, mechanical strength, strong adhesion to steel surfaces, resistance to most pickling acids, ease of application, and adaptation to expanding surfaces. Rubber has been the best behaved material under medium temperatures. Its lifetime may be as much as ten years; however, above 180.degree. F. (82.degree. C.) rubber will rapidly deteriorate. A typical rubber lining may be a 1/8-inch or 3/16-inch thick sheet.
Plastic resins provide a variety of lining materials. However, most are used at temperatures less than 60.degree. C. (140.degree. F.). For example, glass fiber reinforced resins such as epoxy and furane are used as linings and are adhered to the rubber lining of a steel shell. Fiberglass reinforced plastic (FRP) is made of filaments or woven cloth of fiberglass embedded in a plastic resin base, such as a polyester or epoxy resin base. However, a lining of FRP may crack at temperatures greater than 180.degree. F. because the FRP, which is attached to the metal tank, fatigues as the metal tank expands and contracts.
The known designs of linings of carbon brickwork or tile, set in cement over a rubber lining or FRP adhered to a rubber lining on a steel shell do not react in a satisfactory manner to the expanding and contracting surfaces of the metal shell.
U.S. Pat. No. 3,536,601 describes a method which attempts to lessen excessive acid attack during the pickling of iron and steel when the metals spend too much time in the acid bath. A DC voltage is applied between the metal and the acid bath to inhibit excessive acid attack.
U.S. Pat. No. 3,429,792 proposes a step-step method for descaling stainless steel surfaces. First, the steel is electrolytically descaled in an aqueous electrolyte and then pickled with an aqueous solution containing about 6.5-15% of ferric chloride and 5-10% of nitric acid.